The present invention relates to a hybrid bandpass filter using a combination of a longitudinally coupled leaky surface acoustic wave (LSAW) resonator and a network of LSAW impedance-element filters, and more particularly, to a hybrid bandpass filter with improved performance which is manufacturable using standard fabrication techniques resulting from compatible metalization thicknesses of the coupled resonator and impedance-element components.
As the telecommunications industry and society continue to push for mobile communications devices which are smaller, lighter, less expensive, and more energy efficient, the requirements for bandpass filters within these devices become increasingly stringent. Where once transversely coupled surface acoustic wave (SAW) resonator filters were widely used, high-performance transversal SAW filters or longitudinally coupled SAW or LSAW filters have begun to take their place. Transversal SAW filters have the advantages of high flexibility, wide bandwidth, and flat group delay time. However, with newer digital mobile communications protocols requiring smaller size and even less insertion loss, transversal filters simply cannot meet the requirements.
Longitudinally coupled LSAW resonator filters have shown much promise in meeting many of the application requirements because of their wide achievable bandwidth and low insertion loss. Conventional longitudinally coupled LSAW resonator filters typically consist of a plurality of LSAW resonator filter tracks connected in series. Each track generally includes a pair of reflective gratings, between which are disposed a plurality of interdigital transducers (IDTs). In each track, one or more non-adjacent IDTs act together to form a signal input for the track, and the remainder of the IDTs form an output. Adjacent tracks are connected together in series such that the output of the first track is connected to the input of the second, whose output is connected to the input of the third, etc. The input of the first track and the output of the last track comprise the electrical input and output of the bandpass filter. The most common configurations employ only two tracks with two, three, or five IDTs in each track. By way of example, FIG. 2a illustrates a schematic representation of a two-track longitudinally coupled LSAW resonator filter of the prior art with three IDTs per track, and FIG. 2b illustrates one with five IDTs per track. In both cases, the filters include two separate tracks, each with input IDTS 20, output IDTs 21, and gratings 22.
Good bandpass characteristics can be achieved with longitudinally coupled LSAW resonator filters by introducing resonant cavities between adjacent IDTs and between the gratings and the IDTs adjacent to them. As described in U.S. Pat. No. 5,485,052, the resonant cavities are introduced by inserting spacers between each IDT and its neighboring IDT or grating. The length of these spacers can be either positive (i.e. moving the IDTs/gratings further apart) or negative (i.e. moving the IDTs/gratings closer together). Spacers between adjacent IDTs are typically on the order of xc2x1xcex/4, where xcex is the acoustic wavelength, and the spacers between the gratings and the adjacent IDTs is usually much smaller (e.g. xc2x1xcex/40).
Although longitudinally coupled LSAW resonator filters exhibit good passband characteristics and strong rejection of frequencies substantially removed from the passband, they are typically plagued by inadequate rejection of frequencies close to the passband, especially those frequencies just above the passband. This renders them unusable in many applications requiring strong near-in rejection. FIG. 3 demonstrates this phenomenon.
Another technology used prolifically for mobile communications applications includes use of a xe2x80x9cladderxe2x80x9d filter, the name of which comes from its architecture of repeated series-connected and shunt-connected impedance-element filters. Impedance-element filters are simple one-port resonators, as illustrated with reference to FIG. 4. They consist of a simple IDT 23 disposed between two reflective gratings 24. At the resonant frequency of one of these devices, the impedance is extremely low; at the anti-resonant frequency, on the other hand, the impedance is very high. By utilizing these devices as impedance elements in an electrical circuit, various filter characteristics can be achieved. The series connection of such a device acts as a crude low-pass filter with a deep notch corresponding to the anti-resonant frequency where the device""s impedance substantially inhibits the signal from getting through. This Is shown as the thin line (Series) plotted in FIG. 5. The shunt connection of such a device, on the other hand, acts as a high-pass filter, with a deep notch corresponding to the resonant frequency, where the impedance is so low as to short much of the signal to ground. The frequency response in this configuration is plotted with the thick line (shunt) in FIG. 5. Hence, by repeated series-shunt combinations, or xe2x80x9claddersxe2x80x9d, as illustrated in FIG. 6a and shown schematically in FIG. 6b, bandpass filters can be realized with low insertion loss and excellent rejection of frequencies close to the pass band. However, ladder filters suffer from poor rejection of frequencies substantially removed from the passband, as demonstrated in the transfer function plot of FIG. 7.
The idea of combining longitudinally coupled LSAW resonator filters and LSAW impedance-element filters in order to achieve good near- and far-frequencyrejection is not novel. An allusion to this combination can be found in U.S. Pat. No. 5,610,566. However, U.S. Pat. No. ""566 fails to recognize that LSAW impedance-element filters require thick metalization in order to reap the benefits of high reflectivity and high piezoelectric coupling, thereby achieving a high Q. As will be herein described, this requirement renders LSAW impedance-element filters incompatible with conventional LSAW coupled resonator filters at the same relative metal thickness. FIG. 8a and FIG. 8b demonstrate the variation of the piezoelectric coupling coefficient, K2, and the reflectivity, xcexa, as a function of aluminum metalization thickness on 42xc2x0 Y-rotated lithium tantalate (LiTaO3). On that substrate, impedance element filters require aluminum metalization thickness of at least 9% of the acoustic wavelength.
Conventional longitudinally coupled LSAW resonator filters, on the other hand, require a thinner metalization. This is because the velocity of the LSAW mode is in very close proximity to the slow shear bulk acoustic wave (BAW) mode. Whenever a discontinuity is encountered by the propagating LSAW, energy is reflected backwards and, due to the close proximity of the BAW, a significant portion of that energy can be converted into BAW energy and lost into the bulk of the substrate. This energy loss is commonly referred to as xe2x80x9cradiationxe2x80x9d or xe2x80x9cscatteringxe2x80x9d loss. As reflectivity goes up, BAW radiation losses at the discontinuities go up as well. The spacer-type resonant cavities constitute significant phase discontinuities. Thus, conventional longitudinally coupled LSAW resonator filters are usually limited to metalization thicknesses of 8.5% or less. As thickness is increased above that value, losses due to BAW radiation outgrow the gains from increasing piezoelectric coupling and reflectivity.
Thus, in order to achieve a low-loss filter, the combination of the two aforementioned technologies would require different metalization thicknesses for the coupled resonators and the impedance-element filters, despite the suggestion otherwise by U.S. Pat. No. 5,610,566. This would require complicated fabrication steps, thus rendering the device unmanufacturable by reasonable standards. The present invention overcomes this problem by utilizing a novel longitudinally coupled LSAW resonator filter with chirp-type resonant cavities, rather than spacers. This structure exhibits significantly less BAW scattering and is, therefore, able to reasonably operate with a metalization thickness compatible with impedance-element filters. Heretofore, such a combination has never been proposed.
The present invention combines longitudinally coupled LSAW resonator filters with LSAW impedance element filters into a hybrid structure in order to achieve improved out-of-band rejection over either technology by itself. The problem of incompatible metalization thicknesses between the two technologies is overcome by utilizing a novel longitudinally coupled LSAW resonator filter architecture as described copending U.S. Patent Application titled xe2x80x9cLongitudinally Coupled Leaky Surface Acoustic Wave Resonator Filter,xe2x80x9d claiming priority to Provisional Application serial No. 60/286,901 the disclosure of which is herein incorporated by reference. In this approach, the spacer-type resonant cavities of conventional longitudinally coupled LSAW resonator filters are replaced by xe2x80x9cchirpedxe2x80x9d cavities, which are formed by assimilating the spacers into the first one half or more wavelengths of IDTs, as shown in FIG. 9. In so doing, the abrupt phase discontinuity of the spacer is eliminated, thereby significantly reducing the scattering of acoustic energy into BAWs in the cavity. Because of this, the metalization thickness can be increased substantially without excessive radiation loss. Metalization thickness equal to that ideal for the impedance-element filters is easily achievable.
This novel chirp-type longitudinally coupled LSAW resonator filter demonstrates little advantage over typical devices in the art in terms of near-frequency rejection. However, the structure has the distinct advantage that it can be formed on the same substrate along with impedance-element filters, both with the same metalization thickness. This hybrid device can then be fabricated in one photolithographic step, and the resultant filter exhibits much better rejection characteristics than either technology by itself.
By way of example, one embodiment of the present invention comprises first, second, and third IDTs disposed on a piezoelectric substrate and arranged in a surface wave propagating direction such that the second IDT is interposed between the first and the third IDTs, each of the first, second, and third IDTs having a plurality of electrode fingers. In this embodiment, the first and second IDTs have narrow electrode-finger pitch sections which have an electrode-finger pitch narrower than the remaining electrode-finger pitches, at respective end portions of the first and second IDTs, adjacent to each other. Likewise, the second and third IDTs have narrow electrode-finger pitch sections which have an electrode-finger pitch that is narrower than the remaining electrode-finger pitches, at respective end portions of the second and third IDTs, adjacent to each other. The electrode-finger pitch of the narrow electrode-finger pitch sections in the first and second IDTs are different from the electrode-finger pitch sections in the second and third IDTs. The respective narrow electrode-pitch sections also comprise different numbers of fingers with respect to each other. In this embodiment, the present invention further comprises at least one LSAW resonator connected in series or parallel to the longitudinally coupled resonator type LSAW filter.